" This is mixed
with fireclay and used for furnace linings.
Many new forms of refractories have come into use to meet the demands of
the new high temperature work. The essentials are that it should not
melt or crumble at high heat and should not expand and contract greatly
under changes of temperature (low coefficient of thermal expansion).
Whether it is desirable that it should heat through readily or slowly
(coefficient of thermal conductivity) depends on whether it is wanted as
a crucible or as a furnace lining. Lime (calcium oxide) fuses only at
the highest heat of the electric furnace, but it breaks down into dust.
Magnesia (magnesium oxide) is better and is most extensively employed.
For every ton of steel produced five pounds of magnesite is needed.
Formerly we imported 90 per cent. of our supply from Austria, but now we
get it from California and Washington. In 1913 the American production
of magnesite was only 9600 tons. In 1918 it was 225,000. Zirconia
(zirconium oxide) is still more refractory and in spite of its greater
cost zirkite is coming into use as a lining for electric furnaces.
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